1. (Technical Field of the Invention)
The present invention relates to amorphous silica particles having a double structure, to a process for preparing the same and to the use thereof. More specifically, the invention relates to amorphous silica particles having a double structure of a core of the dense amorphous silica and a shell of the bulky amorphous silica, to a process for preparing the same, and to the use such as delustering agent for the coating materials.
2. (Prior Art)
Coating materials have heretofore been blended with a fine silica powder as a delustering agent in order to lower the luster on the surfaces of the coated films. The delustering agent forms fine ruggedness on the surfaces of the coated film to thereby lower the gloss value.
As delustering silica, there has been used a wet-method silica, i.e., a wet-method silica obtained by neutralizing the sodium silicate with an acid. For example, Japanese Patent Publication No. 6669/1980 discloses a delustering composition comprising a fine powdery silica having a refractive index of larger than about 1.448 and an ignition loss of smaller than 2%, that is obtained by treating the ordinary wet-method silica with heat at a temperature of not lower than about 800xc2x0 C.
Japanese Laid-Open Patent Publication No. 298014/1989 discloses a process for producing a delustering agent for coating materials, obtained by pulverizing wet-method silica until its average particle diameter becomes from 1 to 20 xcexcm followed by the heat treatment at a temperature of from 400 to 1000xc2x0 C. for 30 to 120 minutes.
Japanese Laid-Open Patent Publication No. 289670/1990 discloses a delustering agent comprising an inorganic hydrogel having a porous volume of at least 1.0 ml/g, an average particle size of from 1 to 10 microns, and a specific particle size distribution of when it is dispersed in a coating vehicle.
Furthermore, Japanese Laid-Open Patent Publication No. 117548/1993 discloses a delustered coating composition containing a fine silica powder and resin particles having a positive xcex6-potential and an average particle diameter of from 0.01 to 5 xcexcm.
Moreover, Japanese Laid-Open Patent Publication No. 166091/1995 discloses a delustering agent for coating materials obtained by treating the surfaces of the wet-method precipitation silica with a polyethylene wax, and having a BET specific surface area of from 30 to 150 m2/g, an oil-absorbing amount of from 120 to 280 ml/100 g, a bulk density of from 30 to 150 g/l, and an average particle diameter of secondary particles of from 1.0 to 5.0 xcexcm, the amount of the polyethylene wax being from 4 to 12 parts by weight per 100 parts by weight of the wet-method precipitation silica.
The fine silica powder that has heretofore been used as the delustering agent comprises shapeless secondary particles which are formed by the aggregation of primary particles having fine particle diameters. Therefore, the particle shapes are indefinite, particle sizes are widely distributed and the particles are asymmetric. In a state of being dispersed in a coating material, furthermore, it is difficult to control the diameter of the particles that are dispersed.
That is, a predetermined limitation is imposed on the diameter of the silica particles that work to effectively lower the luster by forming ruggedness on the surface of the coated film. The particles smaller than the effective particle diameter are not effective in decreasing the luster on the surface of the coated film, and the particles larger than the effective particle diameter form lumps on the surface of the coated film to deteriorate the appearance and surface properties of the coated film. Besides, in the conventional silica-type delustering agent, the content of the particles larger than the above-mentioned effective particle diameter can be decreased by pulverization or classification accompanied, however, by an increase in the content of the particles having diameters smaller than the effective particle diameter, causing inconvenience to take place as described below.
First, the content of the particles having the effective particle diameters helpful for decreasing the luster on the surface is small and, hence, the amount of blending the silica-type delustering agent must be increased per the coating material, requiring an increased cost and cumbersome blending operation. Moreover, with the particles having diameters smaller than the effective particle diameter being contained in the coating material, the viscosity of the coating material increases causing the efficiency of coating operation to decrease. Besides, the presence of particles of small diameters in the coating material causes the hue of the coated film to become dark and dull, and further causes mechanical properties such as toughness of the coated film to be deteriorated. Furthermore, the coated film containing aggregated particles of small diameters tends to be scarred on the surface due to friction.
The conventional shapeless silica-type delustering agent may have a particle constitution that is satisfactory to some extent before being blended. When dispersed in the coating material, however, the conventional shapeless silica-type delustering agent tends to be collapsed (disintegrated) due to the shearing force, and are not capable of avoiding the above-mentioned defects.
Amorphous silica has another defect in that it causes abrasion and gives scars; i.e., the device for handling the coating material is abraded, and the coating material gets scarred when it is abraded. The slipping property can be improved when the surfaces of the amorphous silica particles are treated with a polyethylene wax or the like as proposed in the foregoing without, however, radically solving the above-mentioned problems.
Such defects of the amorphous silica are not confined simply in the delustering agent for coating materials but also occur similarly even when the amorphous silica is used as an antiblocking agent for the resin films.
It is, therefore, an object of the present invention to provide amorphous silica particles having a novel structure free from the above-mentioned problems, maintaining constant particle diameters not only in the powdery state but also in a state where they are blended in a coating material or in a resin, without developing fine particles, exhibiting excellent delustering action and antiblocking action, exhibiting decreased abrading property, and exhibiting excellent resistance against scars or abrasion.
Another object of the present invention is to provide a method capable of easily preparing the amorphous silica particles having the above-mentioned novel structure maintaining excellent productivity and economically.
A further object of the present invention is concerned to the use of the amorphous silica particles having the above-mentioned novel structure and, particularly, to provide a delustering agent for coating materials and an antiblocking agent for films.
According to the present invention, there are provided amorphous silica particles having a double structure of a core of the dense amorphous silica and a shell of the bulky amorphous silica, having a volume-based median diameter (average particle diameter) of from 1 to 5 xcexcm and containing not more than 10% by volume of fine particles having a particle diameter of not larger than 0.5 xcexcm.
In the amorphous silica particles of the present invention, it is desired that:
1. a BET specific surface area is from 150 to 400 m2/g and a BET porous volume is from 0.2 to 2 ml/g;
2. the cores of the dense amorphous silica and the shells of the bulky amorphous silica are contained at a weight ratio of from 2:8 to 7:3 and, particularly, from 3:7 to 5:5;
3. the cores of the dense amorphous silica are gel-method amorphous silica particles, and the shells of the bulky amorphous silica are precipitation-method amorphous silica particles precipitated on the surfaces of said core particles;
4. the cores of the dense amorphous silica has a BET specific surface area of from 200 to 800 m2/g, a BET porous volume of from 0.2 to 2 ml/g, and a porous volume distribution in which a porous volume of pores having radii of from 10 to 150 angstroms is from 40 to 80% of the whole porous volume.
According to the present invention, furthermore, there is provided a process for preparing amorphous silica particles having a double structure comprising:
a step of neutralizing an alkali silicate aqueous solution and a mineral acid aqueous solution under a condition of a pH of from 2 to 10 to prepare gel-method amorphous silica;
a step of wet-pulverizing the thus formed gel-method amorphous silica; and
step of neutralizing the alkali silicate aqueous solution and the mineral acid aqueous solution in the presence of the wet-pulverized gel-method amorphous silica particles under a condition of a pH of from 5 to 9, so that the precipitation-method amorphous silica particles are precipitated on the surfaces of the gel-method amorphous silica particles.
In the process of the present invention, it is desired that:
1. the gel-method amorphous silica particles are prepared at a temperature of not higher than 50xc2x0 C.;
2. the gel-method amorphous silica is wet-pulverized so that the volume-based median diameter becomes from 0.5 to 3 xcexcm; and
3. the precipitation-method amorphous silica particles are precipitated at a temperature of from 60 to 100xc2x0 C.
According to the present invention, furthermore, there are provided a delustering agent for coating materials and an antiblocking agent for films, comprising the above-mentioned amorphous silica particles.